The first apparatus to commercially produce fuel gas from coal began to operate in the early 1800s. The equipment was simple, primarily consisting of a horizontal retort through which air or air and steam mixture was passed over a bed of incandescent coal. The products from the process were gas, tar and coke. About a hundred years later, the market no longer needed a large amount of coke. Consequently, vertical retorts were introduced to completely convert coal to ash.
These vertical retorts, now known as single stage gasifiers, received coal from the top and air and steam at the bottom. Five chemical and mechanical processes then took place in the gasifiers. First, there was the process of drying the coal. Second, the distillation of volatile matters occurred. Third, the gasification reaction took place. Fourth, the residual carbon in the coal was burned, and fifth, the residual ash was removed through a water sealed revolving grate.
The drying and distillation were driven by the heat contained in the gas as it rose through the descending coal. Control of the drying and especially the distillation was difficult, especially with coals with high volatile content. As a means to solve this problem, a two-stage gasifier design was created in the mid 1900s, in which the drying and de-volatilization processes were separated from the gasification process.
This “two-stage” design fitted a second stage on top of a single stage gasifier so that the gasification could take place in the lower stage and drying and distillation in the upper stage. A refractory system including passageways was also fitted into the upper stage which allowed a portion of the gas produced in the lower stage to flow to an exit near the top of the upper stage. The remaining portion of the gas passed through the coal in the upper stage. The relative flows of the two portions were controlled by valves in the outlet pipes. An exemplary two-stage gasifier in the prior art is illustrated in FIG. 1 herein.
To prevent gas from leaking through the gasifier when discharging ash from the gasifier, water was used at bottom of the gasifier as a seal. The entire unit rotates slowly on a bearing surface located on a support structure below and there is no connection between the main body of the gasifier and the grate unit. Therefore, the gasifier in the prior art could only operate at atmospheric pressure or slight higher than atmospheric pressure. The output was low and the coal moved very slowly through the gasifier. As a result, a large number of gasifiers must be installed if the fuel gas was used to feed gas turbines, which generally required a delivery pressure of 250 to 450 psia. This is burdensome to operate and costly to maintain. Nevertheless, the “grate and water seal” design has been used for more than 50 years with very little changes, largely due to the difficulties in designing a vessel that can sustain the high pressure and the cost of constructing one.
The slow movement of the coal through the gasifier also made it difficult to know the exact condition of the process occurring in the gasifier. To solve this problem, U.S. Pat. No. 4,134,738 provided a system which used a rod to poke into the lower stage of the gasifier for a few minutes and then retrieve the rod for visual observation. This procedure was complicated and did not help to improve the output of the gasification process.
Another problem associated with the prior art gasifier was the tendency for the coal to swell and block the descending lanes of the reaction vessel due to the slow movement of the coal from the top to the bottom of the vessel and the rapid rate of heating applied to the coal. One solution was provided by U.S. Pat. No. 3,454,383 which used stirrers for the whole depth of the gasifier. However, this design eliminated many useful features of the original design as in the Wellman Incandescent Ltd Leaflet (FIG. 1 of the current application) and U.S. Pat. No. 4,134,738.
To solve one or more problems associated with the prior art gasifier, the current invention presents the following embodiments.